Insulating thin films of silica are known for use as protective layers and as electrically insulating layers in electronic devices. The wet-coating of compositions comprising resin and solvent is generally and widely employed in this field. For example, U.S. Pat. No. 4,756,977 teaches a process for coating electronic devices with a silica thin film by applying a solvent solution of hydrogen silsesquioxane resin on a substrate, evaporating the solvent, and heating at temperatures of 150.degree. C. to 1,000.degree. C. to effect conversion into ceramic-like silica. Lower dielectric constants have been required from these insulating layers as devices have shrunk in size and become more highly integrated. For example, a dielectric constant of 2.7 has been reported for a thin film afforded by the cure of hydrogen silsesquioxane (43rd Extended Abstracts of the Japan Society of Applied Physics and Related Societies, page 654, Abstract 26a-N-6). However, dielectric constants below 2.7 are required of the electrically insulating thin films in the next-generation highly integrated circuits with design rules narrower than 0.18 .mu.m. Nevertheless, to date no process has been discovered for the stable and highly reproducible generation of such low dielectric constants in the electrically insulating thin films suitable for highly integrated circuits, for example, in silica films.
On the other hand, it is known that the insulating thin film can itself be executed as a porous structure in order to reduce the dielectric constant of the insulating thin film. For example, U.S. Pat. No. 5,548,159 has reported the formation of an insulating thin film with a porous structure through the use of the baked product of hydrogen silsesquioxane resin as the dielectric layer in a highly integrated circuit. U.S. '159, however, does not disclose a specific method for the formation of the porous structure. Therefore, a composition and process capable of forming an electrically insulating thin film with a low dielectric constant (a dielectric constant materially below 2.7) is not known.
It is an object of this invention is to provide a composition and process that can produce an electrically insulating thin film with a low dielectric constant, specifically with a dielectric constant below 2.7.